Ionization and plasma acceleration apparatus



Dec. 8, 1964 D. l.. CUR-ns 3,159,966

' IONIZATION'AND PLASMA ACCELERATION APPARATUS Filed Nov. 7. 1960 @Pos1 United States Patent O 3,159,965 IONIZATION AND PLASMA ACCELERATN APPARATUS Daniel L. Curtis, Manhattan Beach, Caiif., assigner to Litton Systems, Inc., Beverly Hilfs, Calif. Filed Nov. 7, 1960, Ser. No. 67,579 7 Claims. (Ci. oil- 355) The present invention relates to apparatus for ionizing gas and for accelerating the resultant gas plasma.

The apparatus disclosed and claimed in the present application is, from one aspect, an improvement on the structure disclosed in the joint patent application of Siegfried Hansen and myself, Serial No. 67,645 entitled Ionization and Plasma Accelerator Apparatus, filed concurrently with the present application.

Apart from the apparatus disclosed in the joint application noted above, a number of systems have been-proposed heretofore for ionizing gas and for accelerating it. In this regard, several plasma acceleration systems are described in the article entitled A Comparison of Ion and Plasma Propulsion by S. W. Kash which appeared at pages 45 8 through 465 of lthe Proceedings of the Institute of Radio Engineers, April 1960, volume 48, No. 4. In the electrostatic systems disclosed in this article, the electrodes are in contact with gas discharge, and therefore tend to be eroded and ejected as part of the fuel. In addition, these systems are generally not adapted for continuous operation.

Other systems involving the use of coils have been proposed, one such system being disclosed in copending application Serial Number 723,018, of Siegfried Hansen, now Patent Number 2,992,345, granted July l1, 1961 which is assigned to the assignee of the present patent application. This system includes circuitry and coil structure for providing an accelerating eld to accelerate the gas plasma.

In the apparatus disclosed in the joint application noted above, a flared or funnel-shaped coil is used. High frequency single phase signals may he applied across the coil to both ionize gas supplied to the apex of the flared coil, and to forceably eject 4the resultant plasma from the coil.

In many of the plasma acceleration devices discussed above, it is important to obtain a high degree of ionization of the gas employed as the propellant of the propulsion system. Accordingly, a principal object of this invention is to increase the ionization of gas in plasma acceleration devices. p v

In accordance with the present invention, either or both of the following two improved types of construction may be employed to increase the gas ionization, First, in the case of any of the acceleration devices in which a multiturn coil is employed, lthe turns of the coil may be closely spaced together in the region where the gas enters the coil; in the case of the flared or funnel-shaped coil construction mentioned above, this wouldbe near the apex of the coil. The additional turns provide a relatively high intensity field at the point where the gas is introduced into the coil and therefore tend to produce relatively higher ionization.

As discussed in detail below, it is considered that free electrons play a predominant role in the ionization of the gas. In accordance with another aspect of the invention, therefore, apparatus is provided for greatly increasing the number of free electrons available in the ionization zone. In one specific embodiment of the invention, this is accomplished by the use of a tungsten filament which may be energized by an external current source. In another, preferred, embodiment of the invention, tungsten material having an extended surface area is provided near the gas input to the acceleration structure. The tungsten ice material becomes heated to an elevated temperature at which it gives off many electrons, and the resultant electron cloud permits greatly increased ionization of the gas. Furthermore, the combination of the closely-spaced turns and the increased number of electrons provided by the tungsten material produces a much higher degree of ionization than has been possible up to the present time.

In accordance with a specific feature of the invention, therefore, a flared or funnel-shaped coil has its turns more closely spaced toward the apex of the coil and also includes apparatus for providing free electrons near the apex of the coil Where gas is introduced.

In accordance with another feature of the invention, an electromagnetic plasma accelerator is provided with a multiturn coil in which the turns at one end are more closely spaced together than at the other end, apparatus is provided for introducing gas into the coil at the end where the turns are closely spaced together, and electrical circuitry is provided for energizing the coil with single phase high frequency signals.

In accordance with another aspect of the invention, the ionization zone of a plasma acceleration apparatus is provided with a broad area of electron-emitting material, such as tungsten, for enhancing gas ionization.

Another problem encountered in the use of coil-type plasma accelerators is destruction through overheating.

Related matters involve the prevention of gas leakage between the turns of the coil, and the support of spaced turns ofthe coil under conditions of thermal and mechanical shock.

In accordance with a further feature of the invention, the foregoing difficulties may be overcome through the use of a plasma acceleration coil made of hollow conductive tubing which is imbedded in a refractory material, such as quartz cement. With this arrangement, coolant circulated through the coil carries away heat absorbed at the surface of the refractory material facing the plasma.

Another interesting feature of the invention involves the contributions of the improved ionization structures toward overcoming the heating problem. In this regard, it is understood that the ionized plasma is acted upon by the fields produced by the coil to accelerate the plasma away from the funnel-shaped refractory ma trix in which the coil is imbedded. Thus, it is the gas which is not ionized which `strikes the inner surface of the refractory material and heats it up. Accordingly, with the improved ionization provided by the closelyspaced turns and the tungsten material as discussed above, the heating is reduced, for any given level of thrust from the accelerator. Thus, the improved ionization structures cooperate with the last mentioned features of thev externally cooled coil imbedded in the refractory material to provide a powerful accelerator which operates without overheating. y

Other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description and from the drawing in which:

FIG. 1 is a schematic diagram of one form of ionization and plasma acceleration device in accordance with the invention;

FIG. 2 is an alternative preferred embodiment of the invention; and

FIG. 3 is a diagram indicating one form of electrical circuit which may be employed in the energization of the funnel-shaped coil. FG. 3 also indicated schematically the use of the plasma accelerator of the present invention in a space vehicle.

With reference to the drawing, FIG. l shows an ernhodiment of the invention in which the coil 14) is imbedded in a funnel 12 of refractory material. The funnel 12 may, for example, be of a quartz cement which retains its strength and integrity to about 2500 F.

The coil 10 is arranged with a large number of closelyspaced turns 14 at its apex. This produces high field strengths at this region where the gas is to be ionized. Toward the flared mouth of the funnel-shaped accelerator, the turns of the coil are spaced further apart.

The gas to be ionized is supplied to the coil l through tube 16. To supplement tl'ie ionizing action of the closelyspaced turns 14, a tungsten wire or filament 1S is provided. The tungsten filament is twisted without short circuiting to avoid strong steady magnetic fields which might reduce ionization. The filament 15 supplies many free electrons which assist in ionizing the gas supplied through tube 16. The connections Ztl to the tungsten filament 18 pass through the vacuum sealing wall 22 and are connected to the filament current source 24. The support member 26 is provided to supplement the terminal conductors 28 and 39 of the coil 10 in holding the coil structure rigidly in position. input and output conductors 28 and 36 are preferably of hollow conductive material. This permits water cooling by connection to the coolant supply and radio frequency isolation apparatus 32. Normally, a rubber or plastic tube, five or ten feet in length, is suiiicient for radio frequency isolation when tap water is used for cooling. High frequency signals are supplied to the coil by the radio frequency power source 34, and the insulating tube included in block 32 of FIG. 1 avoids short-circuiting of the high frequency source 34 through the coolant system.

The arrangement as shown in FIG. 2 has several distinct advantages over previous proposals, as mentioned in the introduction of this specification. Initially, with the turns 44 being more closely spaced near the apex of the funnel, the incoming gas is more readily ionized.

In addition, the tungsten filament is helpful in providing the free electronswhich are desirable for rapid ionization of the incoming gas. The use of a construction in which the water-cooled coil 10 is imbedded in the refractory material is also advantageous, from a heat-absorption standpoint. Thus, the water-cooled coils tend to carry away heat generated in the gas and transmitted, principally by gas which is not ionized, to the inner surfaces of the refractory funnel 12.

With regard to the radio frequency power source 3d, this may be operated either on a continuous or on a pulsed basis. When continuous operation is employed, the power level is approximately 6 kilowatts, at a frequency between l and 2 megacycles. A duty cycle in which the power source was successively pulsed On for 1 millisecond and then Ofi for 9 milliseconds has also been tested. During the pulsing period, the power is approximately 25 kilowatts. The average power over the entire duty cycle is about 2.5 kilowatts.

The nature of the electric and magnetic fields within the funnel plays an important part in the gas ionization and in the plasma acceleration operation of the apparatus of FIG. 1. More specifically, the magnetic field in the center of the funnel is generally axial, while the electric field patterns are circular and coaxial with the funnel. Thus, the initial electrons generated by the tungsten filament 1S within the funnel are accelerated by the electric field toward a circular trajectory; the axial magnetic field, however, imparts inward and outward components of motion to the electrons, as the direction of the magnetic field reverses.

These inward and outward radial forces on the rotating electrons are a result of the fmotor forces of a magnetic Vfield on a curren flowing perpendicular to the field. In the present case, of course, the current is the electron stream. The alternate inward and outward pulsing forces on the electrons produce a contracting and expanding cloud of electrons which ionizes the gas introduced into the apex of the funnel through collisions. The probability of collisions is increased by the action of the contracting The coil lill and its electrons on the gas in the region of highest density near the apex of the funnel. Additional electrons will, of course, be generated along with heavier charged particles. However, it is the high speed electrons which are believed to play a predominant part in the ionization of the incoming gas to form the desired gas plasma.

The flaring configuration of the coil provides an inner zone near the apex of the coil where the magnetic field diverges gradually and is of high density, and an outer region near the mouth of the funnel where the field diverges rapidly. The high density region permits relatively complete ionization, while the highly divergent field accelerates the plasma from -the mouth of the coil..

ln the operation of the apparatus of FIG. l, gas was supplied through the stem 22 to the apex of the coil at a rate of about 2x10-3 grams per second. The funnel was discharged into an evacuated space in which the ambient pressure was approximately 4X10-'4 millimeters of mercury. In the tests of the device of FIG. l and other similar constructions, a visible plasma could be seen for approximately two meters from the mouth of the funnel.

Through the use of several photographs with probes in various positions, in which the camera was triggered by onset of ionization, the velocity of the plasma was measwhere ISp is the specific impulse, V is the velocity of ejected propellant, and g is the acceleration of gravity.

In the present case, where the velocity of the plasma is about 5X l0'1 meters per second and the acceleration of gravity is 9.8 or approximately 10 meters per second per second, the resultant specific impulse would be about 5,000 seconds, assuming percent ionization. This is in comparison With the specific impulse of the better chemical fuels which may reach 300 or 400 seconds.

The optimum specific impulse depends on many factors. Thus, for example, the energy required to eject fuel is roughly proportional to 1/.MV2, where V is the velocity of the fuel and M is its mass. Thus, to increase the specific impulse by a factor of two where the specific impulse is a direct function of velocity, the power input must be quadrupled. In general, however, it has been determined that, with currently available power supplies, a specific impulse of between 5,600 and 10,000 seconds represents approximately optimum operating conditions, for presently contemplated space missions.

FIG. 2 represents an alternative embodiment of the arrangement of FIG.l 1. In the structure of FIG. 2, the coil 10 and the refractory funnel 12 are of substantially the same configuration as in FIG. l, and these elements,

Y therefore, bear corresponding reference numerals.

At the apex of the coil 10, however, Where the ionizable gas emerges from the tube 16, a different pre-ionization structure is employed. This pre-ionization structure includes a spark gap between the ends of the wires 40 and 42, `and a series of short tungsten wires 44 which are secured to the inner surface of the high silica glass tube 46 to provide an electron-emitting surface of large area. The high silica tube 46 was notched at one end at about l0 points around its periphery yand l0 corresponding short pieces of tungsten wire were bent and cemented into the notches, at the left-hand end of the sleeve 46 as it appears in FIG. 2. Each of the tungsten wires was then cemented at itsV other end so that it extended down'the inner surface of the tube V46. The assembly, including tungsten wires 44 and the tube 46, was then cemented in place within the refractory funnel 12 so that it enclosed the end of the tube 16.

In operation, two distinct modes of discharge were observed. Dun'ng the first stage of operation before the tungsten wires 44 became heated up, the ionization was solely a result of the pre-ionization spark gap between wires 40 and 42 and the action of fields generated by the closely-spaced turns 44. After a brief period of time, perhaps 5 or 10 seconds, however, the tungsten wires 44 became heated to the point where electrons 'were given off in quantity. At this point a significant increase in ionization of Ithe gas greatly increased the loading on the coil 10. In fact, metering circuits showed that the power supplied to coil 10 approximately doubled.

In observing the change in the nature of the discharge from the funnel-shaped accelerator, during the first stage a diffuse gas discharge emanated from the mouth of the apparatus. Following the lapse of a few seconds, the discharge became more concentrated and changed in character from a bright diffuse glow to a duller and more directional beam. This shift in the observed mode of operation coincided with theV doubling of power input mentioned above.

As developed in the preceding paragraphs, the change in mode of operation of the accelerator with heating of the tungsten wires toward incandescence is accompanied by the increased loading and power consumption of the coil, and a visible change in characteristics. These changes show increased ionization of the gas, which significantly increases the eiiiciency of the accelerator.

As the tungsten material in the ionization zone of the apparatus of FIG. 2 was not grounded, the number of electrons was limited in View of the positive potentials of the wires. Thee wires may be connected to ground, if suitable insulation and spacing is provided between the tungsten wires 44 and the turns of the coil l0. As contrasted with the filament 18 of FIG. l, which is heated by an external source, the tungsten structure of FIG. 2 is passive.

The wires 44 may also be extended to the right as shown in FIG. 2 along the walls toward the open end of the accelerator. The additional extent may be equal to or double the length shown in FIG. 2. With this arrangement, the eects of increased ionization and protection of the inner walls of the accelerator are extended well beyond the immediate ionization zone. v

The diagram of FIG. 3 indicates the nature of radio frequency power circuit mentioned above in connection with FIG. l, in somewhat greater detail. In addition, it shows a transformer coupling arrangement for the funnel-shaped acceleration coil. In FIG. 3, the transformer Stl is coupled through the vacuum sealing plate 52 to the funnel-shaped coil 54. The transformer coupling arrangement reduces the high peak-to-peak voltages between the coil assembly 54 and the vacuum isolation plate 52, and permits floating of the coil with respect to plate 52, or other vacuum-tight enclosures or sealing structures with which the coil may be utilized.

The primary winding of the transformer Sti is connected to a 400G-volt supply by the lead 57. Transformer 50 is tuned to the desired frequency range by capacitor 58 which is connected across the primary winding. Driving the primary of the transformer 5ft are the signal generator 60, the pulser 62, and the radio frequency ampliiier 64. The high voltage power supply connected to lead 76 includes output storage capacitance. This accounts for the significantly higher power level for pulse operation than for continuous operation (with pulse 62 disabled) as mentioned above.

The lines 66 represent any known form of space ve- `hicle such as a satellite, rocket, or the like. Thus, the

plasma acceleration system of the present invention is suitable for use in the propulsion of the satellites which are now encircling the earth or of other forms of space vehicle. It is contemplated that it would be used to supplement other more powerful forms of engine which would be employed to lift the vehicle 66 from the surface of the earth into orbit or into space flight, where the more economical propulsion system of the present invention would advantageously be employed.

As'mentioned above,'the present ionization and plasma acceleration apparatus is primarily intended for use as a space vehicle propulsion unit. However, it may also be employed for other purposes in which high velocity gas streams and moderately high volumetric ilow rates are desired. One such application'is a supersonic wind tunnel for simulating conditions at very high altitudes.

With respect to pressures and voltages to which the present system is applicable, the foregoing description has been on the basis of conditions approaching a vacuum and a power input of up to about 25 kilowatts. With higher power levels, operationcould be successfully carried on at somewhat higher pressures.

For completeness, it is also desirable to define the term plasrnaj which is frequently employed in the present specification and claims. As herein utilized, the term plasma designates a volume of gas in which an appreciable percentage of the atoms are ionized, but which contains the detached electrons within the same volume so that the gas as a whole is electrically neutral. Except for mechanical rigidity, therefore, a plasma is very similar to a metal in its properties since the detached electrons remain uncombined for relatively long periods and can move about at random within the boundaries of the gas volume. In fact, it can be readily demonstrated that the electrical conductivity of a relatively highly ionized plasma can actually exceed that of such good electrical conductors as copper and silver.

It is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

Thus, by way of example and not of limitation, the novel form of coil construction would be useful in a plasma accelerator to produce increased ionization even without the refractory funnel, as the electric and magnetic fields produced by the coil tend to confine. the plasma to the space within the coil. Similarly, the extended area of tungsten material would be useful in the ionization zone of other plasma accelerators in which -free electrons are accelerated to ionize gas. Accordingly, it is to be understood that the present invention is limited only by the spirit and scope of the appended claims.

What is claimed is:

l. In a space vehicle propulsion system, a flared electrical coil having an input end of reduced cross-section and a wide and unrestricted output opening at the other end, refractory means for both supporting said coil and for preventing the flow of gas through said coil structure, means for introducing ionizable material into said coil at the smaller end thereof, a tungsten-surfaced structure having an extended surface area facing said ionizable material near the apex of said coil, and a radio frequency source connected to said coil.

2. In an ionization and plasma acceleration apparatus, at least one electrical coil, means for introducing ionizable material into said coil near its center, a passive structure including an extended area of electron-emitting material facing said ionizable material near. the point at which the ionizable material enters said coil, and means for energizing said coil with single phase high frequency electrical signals.

3. In an apparatus for accelerating gas plasma to velocities significantly in excess of sonic velocities, at least bedded in said refractory material, means for supplying K ionzable material to the apex of said body, and a high frequencyy power supply connected to said coil.

5` In combination, a space vehicle, a plasma propulsion ysystem mounted in said vehicle, said propulsion system including a ared body of refractory material opening onto the outside of said vehicle, a flared coil of hollow conductvie material imbedded in said refractory material, means for supplying ionzable material to the apex of said body, a high `frequency power supply connected to said coil, and means for supplying coolant to said coil and for isolating said coolant supply from said high frequency signals.

6. A plasma acceleration apparatus comprising:

a coil structure consisting solely of a single coil having a series' of turns of progressively increasing diameter, said coil being open at its larger end to permit the free and unrestrained ejection of high velocity plasma;

a matrix of refractory material supporting said coil;

means for injecting ionized plasma through said coil near its center; and

means for energizing said coil at radio frequencies.

7. In combination, a space vehicle, a propulsion system mounted in said vehiclesaid propulsion system including:

an electrical coil of hollow conductive tubing and having a series of turns of continuously increasing diameter;

a matrix of refractory material supporting said coil;

means for injecting ionized plasma through said coil near its center;

means for passing coolant through said conductive tubing; and

means for energizing said coil at high frequencies.

References Cited in the file of this patent UNITED STATES PATENTS 2,460,175 Hergenrother Jan. 25, 1949 2,776,391 Peek Jan. 1, 1957 2,798,181 Foster July 2, 1957 2,819,423 Clark Ian. 7, 1958 2,841,726 Knechtli July 1, 1958 2,880,337 Langmuir et al. Mar. 31, 1959 2,992,890 Josephson J an. 26, 1960 3,013,384 Smith Dec. 19, 1961 3,016,693 Jack et al Jan. 16, 1962 3,041,824 Berhman July 3, 1962 OTHER REFERENCES i Engineering publication, October 10, 1958, pages 474 and 475 relied on. 

7. IN COMBINATION , A SPACE VEHICLE, PROPULSION SYSTEM MOUNTED IN SAID VEHICLE, SAID PROPULSION SYSTEM INCLUDING: AN ELECTRICAL COIL OF HOLLOW CONDUCTIVE TUBING AND HAVING A SERIES OF TURNS OF CONTINUOUSLY INCREASING DIAMETER; A MATRIX OF REFRACTORY MATERIAL SUPPORTING SAID COIL; MEANS FOR INJECTING IONIZED PLASMA THROUGH SAID COIL NEAR ITS CENTER; MEANS FOR PASSING COOLANT THROUGH SAID CONDUCTIVE TUBING; AND MEANS FOR ENERGIZING SAID COIL AT HIGH FREQUENCIES. 